Traditionally, powered wheelchairs have two drive wheels at the rear and two castor wheel, at the front. These chairs drive like a car in that they pivot about the rear of the chair. Accordingly, such motorized wheelchairs are generally adapted for movement either along a straight line or along a steered curved arc, somewhat in the same manner as an automobile. They require a significant turning radius and, as with a car, many manoeuvres must be executed backwards. Further, these chairs tend to lose traction on downward slopes since the rear drive wheels tend to become unloaded.
If it is desired to realign the wheelchair, for movement from one fixed position in an entirely new direction, it is typically necessary to go through complex turning manoeuvres, somewhat similar to the three point turn utilized on occasion in operating an automobile. The manoeuvres require a significant amount of space and many tight spaces must be approached backwards in a manner similar to a car reversing into a parking spot. These complex manoeuvres are sometimes difficult for disabled persons to carry out.
There are some powered chairs where the powered drive wheels are at the front. However, these chairs also require a large turning radius and tend to lose traction when going uphill.
In addition, existing motorized wheelchairs frequently have difficulty in traversing uneven flooring or terrain or when travelling along an incline, such as a wheelchair ramp, and may lose traction
In U.S. Pat. No. 5,445,233, (Fornie et at.), the free running wheels were urged into contact with the ground via a spring mounted around the shaft. Accordingly, for example, if the wheelchair of Fernie et al were travelling in a forward direction and encountered uneven terrain, the front wheels could pivot upwardly or downwardly about the shaft while ensuring that the drive wheels remained in contact with the ground. The spring would continually urge the free running wheels into engagement with the ground to stabilize the wheelchair and thereby prevent the wheelchair from tipping over.
One disadvantage with this approach was that the free running wheels would rock forward to backward and/or side to side as a person sat down or stood up from the wheelchair. This produced a sensation that the wheelchair was unstable and would cause concern to a disabled person. This was undesirable as midwheel drive chairs are garnering a reputation as being unstable.
In order to enable the occupant of the chair to reach objects positioned on a low shelf or on the ground, the chair included a shaft having two telescoping sleeves. One of the sleeves was connected to the seat of the chair and the other was connected to the drive wheels. A motor was used to extend or retract one of the telescoping sleeves thus raising or lowering the seat of the chair. While this design achieved the goal of allowing a person to reach low lying objects, it has several disadvantages. First, it necessitated the use of complex gearing and a the incorporation of a further motor into the design of the wheelchair. This constituted additional parts which were subject to wear and tear and potential failure. Further, the parts substantially added to the cost of the wheelchair thereby restricting the ability of some disabled people to acquire the wheelchair.